feat(config): support ADK built-in and custom tools in config

src/google/adk/agents/base_agent.py

@@ -41,6 +41,7 @@
 from ..utils.feature_decorator import working_in_progress
 from .base_agent_config import BaseAgentConfig
 from .callback_context import CallbackContext
+from .common_configs import AgentRefConfig
 
 if TYPE_CHECKING:
   from .invocation_context import InvocationContext
@@ -503,11 +504,13 @@ def from_config(
 
     Args:
       config: The config to create the agent from.
+      config_abs_path: The absolute path to the config file that contains the
+        agent config.
 
     Returns:
       The created agent.
     """
-    from .config_agent_utils import build_sub_agent
+    from .config_agent_utils import resolve_agent_reference
     from .config_agent_utils import resolve_callbacks
 
     kwargs: Dict[str, Any] = {
@@ -517,9 +520,7 @@ def from_config(
     if config.sub_agents:
       sub_agents = []
       for sub_agent_config in config.sub_agents:
-        sub_agent = build_sub_agent(
-            sub_agent_config, config_abs_path.rsplit('/', 1)[0]
-        )
+        sub_agent = resolve_agent_reference(sub_agent_config, config_abs_path)
         sub_agents.append(sub_agent)
       kwargs['sub_agents'] = sub_agents
 

src/google/adk/agents/base_agent_config.py

@@ -43,6 +43,7 @@
 from ..events.event import Event
 from ..utils.feature_decorator import working_in_progress
 from .callback_context import CallbackContext
+from .common_configs import AgentRefConfig
 from .common_configs import CodeConfig
 
 if TYPE_CHECKING:
@@ -52,66 +53,6 @@
 TBaseAgentConfig = TypeVar('TBaseAgentConfig', bound='BaseAgentConfig')
 
 
-class SubAgentConfig(BaseModel):
-  """The config for a sub-agent."""
-
-  model_config = ConfigDict(extra='forbid')
-
-  config: Optional[str] = None
-  """The YAML config file path of the sub-agent.
-
-  Only one of `config` or `code` can be set.
-
-  Example:
-
-    ```
-    sub_agents:
-      - config: search_agent.yaml
-      - config: my_library/my_custom_agent.yaml
-    ```
-  """
-
-  code: Optional[str] = None
-  """The agent instance defined in the code.
-
-  Only one of `config` or `code` can be set.
-
-  Example:
-
-    For the following agent defined in Python code:
-
-    ```
-    # my_library/custom_agents.py
-    from google.adk.agents.llm_agent import LlmAgent
-
-    my_custom_agent = LlmAgent(
-        name="my_custom_agent",
-        instruction="You are a helpful custom agent.",
-        model="gemini-2.0-flash",
-    )
-    ```
-
-    The yaml config should be:
-
-    ```
-    sub_agents:
-      - code: my_library.custom_agents.my_custom_agent
-    ```
-  """
-
-  @model_validator(mode='after')
-  def validate_exactly_one_field(self):
-    code_provided = self.code is not None
-    config_provided = self.config is not None
-
-    if code_provided and config_provided:
-      raise ValueError('Only one of code or config should be provided')
-    if not code_provided and not config_provided:
-      raise ValueError('Exactly one of code or config must be provided')
-
-    return self
-
-
 @working_in_progress('BaseAgentConfig is not ready for use.')
 class BaseAgentConfig(BaseModel):
   """The config for the YAML schema of a BaseAgent.
@@ -133,7 +74,7 @@ class BaseAgentConfig(BaseModel):
   description: str = ''
   """Optional. The description of the agent."""
 
-  sub_agents: Optional[List[SubAgentConfig]] = None
+  sub_agents: Optional[List[AgentRefConfig]] = None
   """Optional. The sub-agents of the agent."""
 
   before_agent_callbacks: Optional[List[CodeConfig]] = None

src/google/adk/agents/common_configs.py

@@ -21,6 +21,7 @@
 
 from pydantic import BaseModel
 from pydantic import ConfigDict
+from pydantic import model_validator
 
 from ..utils.feature_decorator import working_in_progress
 
@@ -77,3 +78,65 @@ class CodeConfig(BaseModel):
             value: True
     ```
   """
+
+
+class AgentRefConfig(BaseModel):
+  """The config for the reference to another agent."""
+
+  model_config = ConfigDict(extra="forbid")
+
+  config_path: Optional[str] = None
+  """The YAML config file path of the sub-agent.
+
+  Only one of `config_path` or `code` can be set.
+
+  Example:
+
+    ```
+    sub_agents:
+      - config_path: search_agent.yaml
+      - config_path: my_library/my_custom_agent.yaml
+    ```
+  """
+
+  code: Optional[str] = None
+  """The agent instance defined in the code.
+
+  Only one of `config` or `code` can be set.
+
+  Example:
+
+    For the following agent defined in Python code:
+
+    ```
+    # my_library/custom_agents.py
+    from google.adk.agents.llm_agent import LlmAgent
+
+    my_custom_agent = LlmAgent(
+        name="my_custom_agent",
+        instruction="You are a helpful custom agent.",
+        model="gemini-2.0-flash",
+    )
+    ```
+
+    The yaml config should be:
+
+    ```
+    sub_agents:
+      - code: my_library.custom_agents.my_custom_agent
+    ```
+    """
+
+  @model_validator(mode="after")
+  def validate_exactly_one_field(self) -> AgentRefConfig:
+    code_provided = self.code is not None
+    config_path_provided = self.config_path is not None
+
+    if code_provided and config_path_provided:
+      raise ValueError("Only one of `code` or `config_path` should be provided")
+    if not code_provided and not config_path_provided:
+      raise ValueError(
+          "Exactly one of `code` or `config_path` must be provided"
+      )
+
+    return self

src/google/adk/agents/config_agent_utils.py

@@ -24,7 +24,7 @@
 from ..utils.feature_decorator import working_in_progress
 from .agent_config import AgentConfig
 from .base_agent import BaseAgent
-from .base_agent_config import SubAgentConfig
+from .common_configs import AgentRefConfig
 from .common_configs import CodeConfig
 from .llm_agent import LlmAgent
 from .llm_agent_config import LlmAgentConfig
@@ -90,44 +90,48 @@ def _load_config_from_path(config_path: str) -> AgentConfig:
   return AgentConfig.model_validate(config_data)
 
 
-@working_in_progress("build_sub_agent is not ready for use.")
-def build_sub_agent(
-    sub_config: SubAgentConfig, parent_agent_folder_path: str
+@working_in_progress("resolve_agent_reference is not ready for use.")
+def resolve_agent_reference(
+    ref_config: AgentRefConfig, referencing_agent_config_abs_path: str
 ) -> BaseAgent:
-  """Build a sub-agent from configuration.
+  """Build an agent from a reference.
 
   Args:
-    sub_config: The sub-agent configuration (SubAgentConfig).
-    parent_agent_folder_path: The folder path to the parent agent's YAML config.
+    ref_config: The agent reference configuration (AgentRefConfig).
+    referencing_agent_config_abs_path: The absolute path to the agent config
+    that contains the reference.
 
   Returns:
-    The created sub-agent instance.
+    The created agent instance.
   """
-  if sub_config.config:
-    if os.path.isabs(sub_config.config):
-      return from_config(sub_config.config)
+  if ref_config.config_path:
+    if os.path.isabs(ref_config.config_path):
+      return from_config(ref_config.config_path)
     else:
       return from_config(
-          os.path.join(parent_agent_folder_path, sub_config.config)
+          os.path.join(
+              referencing_agent_config_abs_path.rsplit("/", 1)[0],
+              ref_config.config_path,
+          )
       )
-  elif sub_config.code:
-    return _resolve_sub_agent_code_reference(sub_config.code)
+  elif ref_config.code:
+    return _resolve_agent_code_reference(ref_config.code)
   else:
-    raise ValueError("SubAgentConfig must have either 'code' or 'config'")
+    raise ValueError("AgentRefConfig must have either 'code' or 'config_path'")
 
 
-@working_in_progress("_resolve_sub_agent_code_reference is not ready for use.")
-def _resolve_sub_agent_code_reference(code: str) -> Any:
-  """Resolve a code reference to an actual agent object.
+@working_in_progress("_resolve_agent_code_reference is not ready for use.")
+def _resolve_agent_code_reference(code: str) -> Any:
+  """Resolve a code reference to an actual agent instance.
 
   Args:
-    code: The code reference to the sub-agent.
+    code: The fully-qualified path to an agent instance.
 
   Returns:
-    The resolved agent object.
+    The resolved agent instance.
 
   Raises:
-    ValueError: If the code reference cannot be resolved.
+    ValueError: If the agent reference cannot be resolved.
   """
   if "." not in code:
     raise ValueError(f"Invalid code reference: {code}")
@@ -137,7 +141,10 @@ def _resolve_sub_agent_code_reference(code: str) -> Any:
   obj = getattr(module, obj_name)
 
   if callable(obj):
-    raise ValueError(f"Invalid code reference to a callable: {code}")
+    raise ValueError(f"Invalid agent reference to a callable: {code}")
+
+  if not isinstance(obj, BaseAgent):
+    raise ValueError(f"Invalid agent reference to a non-agent instance: {code}")
 
   return obj